1. Field of the Invention
Aspects the present invention relate to a fusing unit and an image forming apparatus including the same, and more particularly, to a fusing unit that can minimize thermal deformation of a heat radiating body to enhance fusing efficiency, and an image forming apparatus including the same.
2. Description of the Related Art
In general, a fusing unit is employed in an image forming apparatus to form an image on a printing medium. The fusing unit applies heat and pressure to the printing medium to which a developer is applied to fuse the developer on the printing medium.
The fusing unit may be classified as a roller type, which includes a heating roller and a pressing roller, and a belt type, which includes a pressing belt and an endless belt driven by the pressing belt. The roller type employs the heating roller, which has a large thermal capacity and takes much time to be heated by an internal heat source and is therefore suitable for high speed printing. Contrarily, the belt type employs the endless belt which has a small thermal capacity and is suitable for high speed printing. Further, the belt type has a superior thermal efficiency. Recently, the belt type has been actively under development.
As shown in FIGS. 1 and 2, a conventional fusing unit 1 of the belt type includes a pressing roller 10, a belt 20 which is driven by the pressing roller 10, a pair of bushings 70 which rotatably supports opposite end parts of the belt 20, a nip plate 40 which is disposed opposite to the pressing roller 10 with the belt 20 being disposed therebetween and forms a fusing nip N, and a heat radiating body 30 which is disposed in a lengthwise or axial (X) direction of the nip plate 40 and contacts the nip plate 40 to heat the nip plate 40. The belt 20 is guided by a belt guide 50 and the heat radiating body 30 and the nip plate 40 are disposed in the belt guide 50.
As shown in FIG. 2, the bushings 70 are elastically pressed toward the pressing roller 10 by an elastic member 60, and accordingly, the nip plate 40 supported by the bushings 70 is also pressed toward the pressing roller 10. The heat radiating body 30 includes a ceramic substrate and a heat radiating pattern which is formed on the ceramic substrate. The heat radiating pattern is provided on a side of the ceramic substrate opposite the nip plate 40. The nip plate 40 is made of metal so that heat from the heat radiating body 30 can be evenly conducted along the lengthwise (X) direction thereof.
FIG. 3A is a graph showing deflection of the heat radiating body 30 and the nip plate 40 in the lengthwise (X) direction. Here, E1 and E2 represent opposite end parts of the heat radiating body 30; and C represents a center part thereof. The nip plate 40 is thermally deformed, which is represented as a solid line A in FIG. 3A, due to the pressing of the opposite end parts thereof by the bushings 70 and the heat conduction from the heat radiating body 30. On the other hand, the heat radiating body 30 radiates heat as power is supplied to the heat radiating pattern, and thus, is thermally deformed, which is represented as a dotted line B in FIG. 3A.
Accordingly, the nip plate 40 and the heat radiating body 30 tend to get close to each other at the center parts (C) thereof and tend to be spaced from each other at the opposite end parts (E1 and E2) thereof. Although the nip plate 40 and the heat radiating body 30 are adhered by an adhesive, heat conductivity from the heat radiating body 30 to the nip plate 40 decreases at the opposite end parts (E1 and E2) by repetitive thermal deformation of the heat radiating body 30. Such phenomenon is illustrated FIG. 3B which is a curve showing temperature distribution of the heat radiating body 30. As shown in FIG. 3B, the temperature of the opposite end parts (E1 and E2) of the heat radiating body 30 is 240° C., which is higher than the temperature of the center part (C) thereof, which is 200° C. The temperature difference between the opposite end parts (E1 and E2) is due to the adhesive force between the heat radiating body 30 and the nip plate 40 weakening at the opposite end parts (E1 and E2); and thus, the heat conduction is decreased at the opposite end parts (E1 and E2) compared to the center part (C). As such, the opposite end parts (E1 and E2) of the heat radiating body 30 are overheated, whereas the center part (C) maintains a normal temperature, which is about 200° C.
As a result, the temperature of the opposite end parts (E1 and E2) of the nip plate 40 is lower than that of the center part (C) thereof, which is the same in the belt 20 which is heated by the nip plate 40. Accordingly, the fusing efficiency decreases at the opposite end parts of the belt 20 where the temperature is relatively low compared with the center part, thereby decreasing the printing quality. Further, the non-uniform temperature along the lengthwise (X) direction causes a limit to an arrangement of a temperature sensor or requires multiple temperature sensors to prevent overheating of the fusing unit.